The pollution of unsafe metals is rising with widespread of
industrial expansion [1]. Living point of view the metal chromium arises
from its outstanding role in pollution in the form of industry and its
toxicity to plants, animals, and microbes [2]. The vast amount of
chromium is introduced into the surroundings throughout various sources
like dyeing in fabric industries, compound manufacture, leather
built-up, and metal plating which affect living organisms and cells [3,
4]. Chromium is in two oxidation states, Cr (III) and Cr (VI)
hexa-valent, which is 600 times more toxic than the trivalent form in
systematic manner [5, 6]. Several investigations have revealed that
bulky sum of chromium (VI) is considered to be necessary for usual
metabolic process [7]. The release into the sewage system causes a
solemn environmental collision and chromium occurs mainly in Cr(III)
form, which is oxidized into chromium (VI) due to the occurrence of
organic compounds [8]. The ceiling stage of Cr (VI) tolerable in
wastewater is 0.05 mg [L.sup.-1] [9]. However, the superior levels of
chromium metals have been found to be toxic to the internal organs
[10,11]. World health organization (WHO) has evidently stated that
chromium (VI) is carcinogenic [9]. The human toxicity includes lung
cancer and liver and gastric spoil [12 ]. Nowadays, the majority of the
industries are facing the tricky problem of discarding of chromium (VI)
in wastewater produced in huge quantity. Hexavalent chromium form
chromate (Cr[O.sub.4.sup.-2]) is considerably more soluble in water than
trivalent chromium Cr(III) [13]. The chromium elimination treatment
includes adsorption process, ion exchange, precipitation, reverse
osmosis, and photocatalysis [14]. Most of these methods require high
capital and recuring expenses and, therefore, they are not suitable for
small-scale industries [15, 16]. Photocatalysis has been attracting
growing interest because it provides a new, promising way to meet the
environmental challenges of energy and sustainability; semiconductor
photocatalysis has conventional much attention because of its high
efficiency without inferior pollution [17]. In recent advances in
efficient photocatalytic systems, the synthesis of high-performance
photocatalysts with good recyclability is the one of the most important
purposes; deplorably, the broad band gap of metal oxides confines the
reasonable maintenance under visible light [18]. Moreover, the recovery
of photocatalysts requires tedious steps during recycling. Therefore,
recent research works have focused on developing magnetically separable
visible light responsive photocatalysts to carefully use sunlight [19,
20]. Most of the researchers used [Fe.sub.2][O.sub.3] as a magnetic core
of the nanoparticle for effective efficiency and utilization [21, 22].
Hydrothermal methods were usually used to prepare nanoderivatives so as
to effectively be utilized for visible light-driven activity [23]. The
systematic photocatalytic removal of Cr(VI) from wastewater is by using
emerging materials in the batch progression experiments (Figure 19). The
chromium was removed using photocatalyst with UV light and in the gloomy
at diverse pH range. The utmost removal of Cr(VI) was pragmatic in pH 2
and Ti[O.sub.2] showed peak ability for Cr(VI) removal than Ti[O.sub.2]
thin film [24]. To the best of our knowledge, there are no reports on
the structured nanoporous facile [Fe.sub.2][O.sub.3] and its impact on
the photocatalytic degradation of chromium (VI) under visible light. In
the present study, we have effectively synthesized magnetic
[Fe.sub.2][O.sub.3] from hydrothermal method in the deficiency of any
capping agents. The physical properties such as morphology, element
analysis, and surface area of the nanoporous have been studied in
detail, and toxicity of the samples was measured using Mus musculus skin
melanoma cells to clear determination of toxicity of the synthesized
nanomaterial with an appraisal for the degradation of Cr(VI) in
systematic manner under visible light irradiation to be compared with
different emerging materials in this paper (Scheme 1). Therefore, this
organized approach provides a better option upconversion material for
fundamental photoabsorption. Moreover, the stability and durability of
the [Fe.sub.2][O.sub.3] have also been studied via recycling
experiments.

2.2. Hydrothermal Preparation of Nanoporous [Fe.sub.2][O.sub.3].
The nanoporous [Fe.sub.2][O.sub.3] were synthesized using hydrothermal
method. Fe[Cl.sub.3] and NaOH, ammonia (Aldrich, India), 10.14 g
(37.5mol) Fe[Cl.sub.3] x 6[H.sub.2]O, and 7.45g (37.5mmol) Fe[Cl.sub.2]
x 4[H.sub.2]O were dissolved into 25 ml of distilled water. Twenty-five
milliliters of twenty-five percent ammonia was added to the salt
solution under stirring condition at 700 rpm for 2 min right after 15 ml
of mixture was put into a Teflon-lined stainless Morey autoclave, and
the autoclave is heated to 180[degrees]C in an oven and maintained at 12
h reaction time. Temperature plays a vital role in the formation of
well-defined spherical product. Autoclave was naturally cooled to room
temperature, and the precipitates were washed with distilled water and
isolated under magnet. The final products were dried at 60[degrees] C
and were ready for characterization.

[mathematical expression not reproducible] (1)

2.3. Characterizations and Measurements. Magnetic measurement was
accomplished using a vibration sample magnetometer (VSM, ADE EV9 Model).
Brunauer-Emmett-Teller (BET) precise exterior area, aperture volume, and
aperture size allocation of the samples determined by [N.sub.2]
adsorption at 77 K by a Micromeritics ASAP, 2020, physico-adsorption
analyzer. XRF band was recorded by Minipal- 4 benchtop model, with fine
focus X-ray tube, MO target of multilayer monochromator of 17.5KeV for
efficient elemental study. Ultima III Series, RIGAKU, TSX System, Japan,
with Cu radiation (wavelength 1.54[degrees]A) at room temperature was
used for wide-angle X-ray diffraction (XRD) patterns of the
[Fe.sub.2][O.sub.3] recyclability studies. Scanning electron microscopic
(SEM) images of the [Fe.sub.2][O.sub.3] was captured by HITACHI (S-3400
N, Japan) with 10 kV acceleration voltages. The [Fe.sub.2][O.sub.3] was
measured using UV-Vis Spectrophotometer. The spectral analysis of
[Fe.sub.2][O.sub.3] was carried out by measuring the optical density
(OD) using Beckman Coulter, (DU739, Germany) scanning UV-Vis
Spectrophotometer operated at a resolution of 1 nm between 280 and 800
nm. The Energy Dispersive Spectroscopy (EDS) analysis was carried out
using HITACHI (Noran System-7, USA) system close to SEM for the finding
of composite nanoparticles. The particle size distribution (PDS) and
zeta capacity of iron oxide nanoparticle were observed with Microtrac
(USA) element size monitor. The analyzer gives the size dimension and
verification of particle size distribution. The concentration of
profound metals in waste water is calculated with Inductively Coupled
Plasma Atomic Emission Spectroscopy techniques (ICP-AES) using the
Perkin-Elmer Optima 8000, ICP-OES; photocatalytic study was performed on
a Heber-Immersion type photoreactor (HIPR-Compact, p-8/125/250/400) in
an efficient manner and, finally, pH was observed by with an EU-TECH
instrument pH meter. The electron-pore division development was
confirmed by photoluminescence (PL) spectroscopy under an excitation
wavelength of 325 nm.

2.4. Photocatalytic Study. The stock solution of Cr(VI) 1,000 ppm
was equipped by dissolving [K.sub.2]Cr[O.sub.4] (A.R. grade); in the
double-distilled water, batch studies were carried out using chromate
stock solution to 1 L. The absorbance of chromate solution was
calculated. The calibration curve of Cr(VI) was obtained at [lambda]max
= 540 nm. An identical particle size of the photocatalysts was among 120
to 500 [mu] size for photocatalytic dreadful conditions. In every study,
an accurately weighted amount of [Fe.sub.2][O.sub.3] was used in 100 ml
solution by maintaining pH ofhexavalent chromium solution by means of
adding of necessary amount of A.R. grade NaOH and HCl (E. Merck India)
in the different flask at pH 2-7; the flask was sighted in
Heber-Immersion type photoreactor, by steady shaking by magnetic
stirrer. The photocatalyst is alienated from the solution by means of
centrifugation. The removal efficiency of Cr(VI) was amplified through
decline of pH from f 7 to 2. A highest exclusion was established to be
high at pH 2, so pH was finalized at pH 2.

2.5. Photocatalysis of Industrial Effluent. The effluent containing
Cr(VI) ions was taken away with diphenylcarbazide as masking agent
creates the reddish purple color in the solution. The spectral study of
[Fe.sub.2][O.sub.3] was performed by measuring the optical density (OD)
by UV-Vis spectrophotometer operated at a resolution of 1 nm among 280
and 800 nm. The wastewater sample containing Cr(VI) was introduced with
Heber-Immersion type photoreactor. In the test a precisely weighted
quantity of photocatalysts was added in the flask, adjusting pH with
NaOH and HCl in the different flask at pH 2 to 7. The flask was sited
under Heber-Immersion type photoreactor, by steady shaking by magnetic
stirrer. The photodegradation percentage was calculated by the following
expression:

% Photo-degradation efficiency = [C.sub.0] - C/[C.sub.0] x 100 (2)

To authenticate the sorption of Cr(VI) over the [Fe.sub.2][O.sub.3]
nanoporous material in the dark, the tests were carried out under
identical conditions without visible light irradiation. The chemical
oxygen demand (COD) was deliberate via dichromate oxidation method,
subsequent to completion of photodegradation [25].

2.6. Cell Culture and Treatment. Cell culture and treatment of
(B16-F10) cells were placed in tissue culture flasks and grown in
Dulbecco's modified Eagle's medium (DMEM; Gibco, Thermo
Fisher, USA), supplemented through 5% fetal bovine serum (FBS; Gibco,
Thermo Fisher) and 1% antibiotic penicillin/streptomycin mixture (Santa
Cruz Biotechnology, USA). The culture was maintained in a humidified
atmosphere with 5% C[O.sub.2] at 37[degrees]C (Sanyo C[O.sub.2]
Incubator; Sanyo, Japan). In the route of choosing the concentration of
[Fe.sub.2][O.sub.3], a dose-response curve with different concentrations
(0.01, 0.05, 0.1, 0.5, 1.0, and 2.5[micro]g/l) was plotted with
explosion and MTT (2-(3,5-diphenyltetrazol-2-ium-2-yl)-4,5-dimethyl1-1,3-thiazole bromide) test, subsequent to 72 h of exposure. The
concentrations of nanoporous [Fe.sub.2][O.sub.3] worn in the other
experiments were 5, 50, 100, 200, 300, 400, and 500 [micro]g/l for a
time tie of 72 h; the studies were carried out in triplicate and three
self-determining repetitions [26].

2.7. Statistical Analysis. Three replicates were analyzed for each
experiment and by analysis of variance (ANOVA) using SPSS Inc. 16.0.
Significant things of treatments were determined by F values (p [less
than or equal to] 0.05) and Tukey's HSD test.

3. Results and Discussion

The crystallinity and morphology of the synthesized nonporous
materials were characterized by using XRF, SEM and DLS, and PL spectra
analysis. The elements in the nanoporous were verified using EDX and
elemental mapping analysis. The magnetic properties (coercivity (Hc),
saturation magnetization (Ms), and remanent magnetization (Mr)) were
studied via VSM analysis. The surface area and pore volume were
calculated from the BET analysis. The photocatalytic activity was
evaluated for Cr(VI) degradation under visible light irradiation. The
electron-hole severance process in [Fe.sub.2][O.sub.3] was significantly
supported by the PL analysis. The photochemical stability and
reusability of [Fe.sub.2][O.sub.3] by using XRD was also discussed in
detail in this article.

3.1. Characterization. X-ray fluorescence is recorded with fine
focus X-ray tube, MO target of multilayer monochromator of 17.5KeV. The
XRF spectra of [Fe.sub.2][O.sub.3] clearly indicate that the presence of
[Fe.sup.+3] XRF patterns of [Fe.sub.2][O.sub.3] nanoporous materials is
shown in Figure 1.

The diffraction peaks in 6.2 to 7.2 keV and the concentration of
the compound Fe-KA being 153256.1 (cps) are perfectly aligned to the
cubic phase [Fe.sub.2][O.sub.3] and are shown in Table 1.

The diffraction peaks related to other inferior phases of iron
oxide are not detected. The magnetic parameters such as coercivity,
saturation magnetization, and remanent magnetization are shown in Table
2. The Ms value of the nanoporous material is much lower than the other
photocatalytic materials. Material is supposed to be renowned that the
magnetic behavior (Mr) of [Fe.sub.2][O.sub.3] is still retained after
photocatalytic procedure suggesting the suitability of the nanomaterials
for magnetic separation and recovery [27, 28]. The decline in Mr for the
nanoporous material not only reduces the aggregation of the catalyst but
also enhances the reusability.

The nanoporous iron oxide control sample shows only an
insignificant uptake at the squat relative pressure end (P/P0< 0.05)
and a quick uptake with an H-3 type hysteresis loop at high relative
pressure end (P/P0> 0.9), representing that the sample contains
predominantly huge mesopores and/or macropores (average size: 28 nm). In
this sample, the mesopores/macropores should be the inter-nanoparticle
pores resulting from the aggregation of the crystalline
[Fe.sub.2][O.sub.3] nanoparticles. The crystalline [Fe.sub.2][O.sub.3]
nanoparticles in the iron oxide sample have an estimated average
diameter of 16 nm, which is nearly identical to the average size
estimated above significantly shows enhanced surface area (354 [m.sup.2]
[g.sup.-1]) and pore volume (0.29 [cm.sup.3] [g.sup.-1]). Meanwhile,
both surface area and pore volume show slight increases with the
increase of the iron oxide pore. Surface area is an important parameter
to determine the photocatalytic activity of nanoparticles and is
performed by BET surface area analysis, shown in Table 3. A
photocatalyst with a high surface area is likely to absorb more Cr(VI)
and react more rapidly; [N.sub.2] adsorption-desorption isotherms of
[Fe.sub.2][O.sub.3] are depicted. The isotherms of [Fe.sub.2][O.sub.3]
are recognized as type-IV as per IUPAC classification, indicating the
presence of mesopores [29]. Moreover, the isotherm profiles have
[H.sub.2] (P/P0 from 0.4 to 0.9) and H-3 type (P/P0 from 0.9 to 1.0)
hysteresis loops, signifying that there are slit-like pores [30]. The
surface area and pore volume of the nanoparticle are lower than those of
other photocatalytic metals and, in general, surface area is not the
crucial factor that determines the photocatalytic activity of silver
based catalysts [31, 32].

In the present research, [Fe.sub.2][O.sub.3] nanoporous material
clutches the structural and functional groups on the surface,
potentially fastening the metals. A piece of iron oxide has a specific
number of active coordination sites, depending explicitly on the surface
area. Surface of the iron oxides can be very influential for the chance
of the allied metal ions to bind to the surface via specific or
nonspecific adsorption. Adsorption to iron oxide surface is pH-reliant
and slight fluctuations in the pH may engage in recreation in
controlling the adsorption-desorption reactions. The morphology of
[Fe.sub.2][O.sub.3] was carried out by Scanning Electron Microscopy; SEM
images were displayed in Figure 2. [Fe.sub.2][O.sub.3] is mainly
composed of spongy like squared nanoporous and the particle length and
breadth are 73.92[micro]m and 31.90[micro]m, respectively. However, all
the nanoporous materials are closely packed and aggregated. EDX was
carried out to examine the elemental composition of the synthesized
photocatalysts and the results are shown in Figure 3. The peaks
corresponding to Fe and O are clearly observed at their normal energy
levels, showing the elemental composition, atomic percentage, and weight
percentage of the photocatalysts. Consistent with the XRF results, EDX
spectra confirm the purity of the prepared samples.

The histogram of dynamic light scattering analysis for particle
size distribution of controllable composites is depicted in Figure 4.
DLS analysis is regarded as one of the reliable techniques for
evaluating particle size, distribution, and zeta potential of
nanoparticles in solution. The present scrutiny revealed the presence of
particles with an average diameter of 33.70 nm in the aqueous colloidal
solution. The surface charge (zeta potential) of the [Fe.sub.2]
[O.sub.3] nanoporous plays a vital role during the interaction with
other environmental degradation systems. The particles zeta potential
value is 13.9 mV that was entirely determined in the present study.
Nanoparticles with a zeta potential of -10 and + 10 mV have a neutral
charge; when it is greater than +30 mV or less than -30 mV, it is
considered to be strongly cationic and anionic, respectively. Based on
the results, the growth mechanisms of the nanoporous [Fe.sub.2][O.sub.3]
can be predicted.

In Figure 5, the peaks at 619 [cm.sup.-1] were attributed to the
Fe-O bond vibration of the [Fe.sub.2][O.sub.3]. The spectrum showed the
bands at 1390 [cm.sup.-1] and 1130 [cm.sup.-1] correspond to the
out-of-level surface C-H vibration caused by the relic of triethylamine
on the surface of particles and the peaks lying on around 1628.59
[cm.sup.-1] were tentatively assigned to the vibration of C-N bond [33].
The peaks at 3149 [cm.sup.-1] are assigned to the v(N [+ or -] H)
vibrations [34], and peaks at 3448 [cm.sup.-1] were assigned to the O-H
stretching vibration of absorbed water [35].

A predictable loom is working to estimate the band gap energy (Eg)
values [36]. The band gap energy values of [Fe.sub.2][O.sub.3]
nanoporous material were calculated to be 2.03 eV correspondingly.

3.2. Photocatalytic Activity. Sequentially to assess the
photocatalytic activity of the geared-up samples under the visible light
irradiation, the trials were conducted with an initial Cr(VI)
concentration of 9 mM, catalyst concentration of 0.75 g/[L.sup.-1], and
pH 5. The photocatalytic routine of [Fe.sub.2][O.sub.3] was shown in
Figure 6.

pH is the imperative parameter that influences the photocatalytic
activity [37]. Trials were performed at pH values of 3.0 (acidic), 5.0
(basel pH of Cr(Vl)), and the 7.0 (neutral) and 9.0 (alkaline). The
deliberation of heavy metal and catalyst dosage were fixed at 9mM Cr(VI)
and 0.75 g [L.sup.-1] ([Fe.sub.2][O.sub.3]), respectively. The outcome
of pH on the photodegradation of Cr(VI) is revealed in Figure 6. The
photodegradation percentage of Cr(VI) was found to be 60.24%, 82.12%,
65.17%, and 51.27% for pH 03, 05, 07, and 09 in that order. It is
illustrious that the photocatalytic activity is decreased by the
increase or decrease of Cr(VI) solution pH.

The photocatalytic activity of ZnO (viable, 99% purity by particle
size 0.1-4.0 mM) and Ti[O.sub.2] (marketable, Ti[O.sub.2]-[P.sub.25])
was also willful for reference. The study confirmed that 81.11% of
Cr(VI) was degraded by the [Fe.sub.2][O.sub.3] nanomaterials at 120 min
of irradiation time. The degradation percentages of Ti[O.sub.2], ZnO,
and Cu[O.sub.2] are 56.31%, 69.92%, and 71.32%, respectively. The
activity of [Fe.sub.2][O.sub.3] (96.11%) is superior when compared to
the standards in Figure 7. This may be credited toward its
photosensitizing ability.

As per zero point charge ([P.sub.zc]), the external property of
[Fe.sub.2][O.sub.3] is pretentious by the logical change in pH. The
values of [Fe.sub.2][O.sub.3] were found to be 7.3, respectively. It is
appealing to mention that Cr(VI) is a toxic heavy metal. An acidic
environment is helpful to increase the electrostatic lure between the
proton from the catalyst and the toxic heavy metal and thus the
photodegradation is high. At low pH (below 5), the odds for
agglomeration are high, which will decrease the active surface area
available for heavy metal adsorption and photon absorption. At finest
pH, the predominant iron, namely, Fe [(OH).sup.2+], not only forms Fe
(II), the most important catalytic candidates in the photodegradation
reactions, but also produces the further [sup.-]OH dependable for heavy
metal [38]. pH was greater than [p.sub.zc]; the exterior of
[Fe.sub.2][O.sub.3] becomes negatively charged. Consequently, the
negatively charged Cr(VI) molecules are repelled by the catalyst
surface, and this leads to a decrease in the photocatalytic activity.
The extremely alkaline conditions are favorable for the cohort of more
number of less reactive high-valence iron species [33].

3.3. Photodegradation Kinetics of Cr(VI). The Langmuir-Hinshelwood
model was effectively used to investigate the kinetics of Cr(VI)
photodegradation. The photocatalytic experiments were carried out under
optimum reaction conditions [[Fe.sub.2][O.sub.3]= 0.75 g/[L.sup.-1],
Cr(VI) = 9 mM and pH 5]. Figure 8 shows the logarithmic plot of Cr(VI)
concentration as a function of irradiation time. The pseudo-first-order
kinetics experiential rate constant for [Fe.sub.2][O.sub.3] is 1.21 x
[10.sup.-2] [s.sup.-1] is appreciably higher than of Ti[O.sub.2] (4.36
x[10.sup.-3] [s.sup.-1] ) and Cu[O.sub.2] (6.45 x [10.sup.-3]
[s.sup.-1]). Hence, the activity of the [Fe.sub.2] [O.sub.3] nanoporous
material is about 2.4 times higher than that of other verified material
in systematic manner. Negligible photocatalytic activity was shown by
ZnO (2.11 x[10.sup.-3] [S.sup.-1]) in this research.

It is necessary to determine the degree of mineralization of Cr(VI)
during photodegradation. Chemical oxygen demand (COD) is a fundamental
investigation for assessing the superiority of effluents and wastewaters
before release.

It predicts the oxygen obligation of the effluent and is used to
monitor and control discharges and to assess the treatment plant
routine. The fraction change in COD during photodegradation was measured
under optimum reaction conditions [Cr(VI) concentration 9 mM, catalyst
concentration 0.75g [L.sup.-1], pH 5, and irradiation time 180 min]. The
solutions obtained after 180 min of photodegradation showed an important
decrease in COD (59.52%); i.e., after 180 min COD was decreased from 89
mg [L.sup.-1] to 36.14 mg [L.sup.-1]. It has been observed that Cr(VI)
molecules were to some extent degraded to intermediates, and small
fraction was subjected to complete mineralization, persuading radical
scavengers. To further gain insight into the degradation mechanism, the
responsibility of the reactive species on the photocatalytic activity
was determined using a series of scavengers. The experiments were
carried out under optimum reaction conditions (Cr(VI) = 9mM,
[Fe.sub.2][O.sub.3] = 0.75 g [L.sup.-1], and irradiation time = 120 min)
in the company of scavengers (2 mM for 200 mL heavy metal solution) such
as t-BuOH for *OH [39], benzoquinone (BQ) for [O*.sup.2] [40], and
potassium iodide (KI) for holes and *OH [41]. The effect of t-BuOH, BQ,
and KI on the photodegradation proportion of Cr(VI) is shown in Figure
9. It is perceptibly observed that the photodegradation percentage of
Cr(VI) is reduced to 36.79%, 39.75%, 49.69, and 74.11% after the
addition of KI, t-BuOH BQ, and, Blank respectively. The photocatalytic
activity of the nanoporous was surprisingly concealed in the presence of
scavengers, indicating that both [O.sup.2*] and the *OH are
enthusiastically occupied in the photodegradation process.

3.4. Photocatalytic Mechanism. The valence band (VB) and the
conduction band (CB) positions of [Fe.sub.2][O.sub.3] are expected by
using the Butler and Ginley equation [42, 43].

[E.sub.VB] = EVB = x-[E.sup.e] + 0.5 Eg (3)

where x is the fixed electronegativity of the semiconductor,
[E.sup.e] is the energy of free electrons on the hydrogen scale (ca. 4.5
eV), [E.sub.VB] is the VB edge potential, and Eg is the band gap energy
of the semiconductor. Te CB position can be deduced using the following
equation:

[E.sub.CB] = [E.sub.VB] - Eg (4)

The x values of nanoporous [Fe.sub.2][O.sub.3] 3.16, accordingly,
the VB and CB of [Fe.sub.2][O.sub.3], were estimated to be 2.11and 0.23
eV, respectively. Through light irradiation, electrons and holes are
produced in the CB and VB of [Fe.sub.2][O.sub.3]. The photogenerated
electrons can simply drift from the CB of nanoporous [Fe.sub.2][O.sub.3]
under the action of a built-in electric field [44]. The holes will stay
in the VB of [Fe.sub.2][O.sub.3]. These progression forces successfully
extend the duration of charge carriers at the [Fe.sub.2][O.sub.3]
nanoporous face. The electrons accumulate on the CB of nanoporous
[Fe.sub.2][O.sub.3] corresponding with dissolved oxygen to form a super
oxide radical ([O.sup.2*]), and they additionally react with [H.sup.+]
to acquiesce to the hydroxyl radical (*OH). The photogenerated holes on
the VB of iron oxide can unswervingly oxidize the Cr(VI) or react with
[H.sub.2]O to produce hydroxyl radicals (*OH). The metal frame of the
Cr(VI) compound is hurriedly tainted by the reactive species
([O.sup.2*], *OH and [h.sup.+]) generated in this photocatalytic
process.

3.5. Tauc Relation for Finding Out the Band Gap. Tauc relation is
the suitable tool and the appropriate technique to determine the optical
absorption spectrum of a nanomaterial; the absorption coefficient a for
material is given by

[alpha]h[gamma] = A [(h[gamma] - Eg).sup.n] (5)

The optical band gap was expected by plotting the straight line of
[([alpha]h[gamma]).sup.2] v/s photon energy as shown in Figure 10.
Bringing to a halt the tangent to the plot provides a good estimate of
the band gap energy of nanoporous [Fe.sub.2] [O.sub.3]. The band gap of
the hydrothermally synthesized nanoporous [Fe.sub.2][O.sub.3] was
investigated that 3.16 eV. nanoporous [Fe.sub.2][O.sub.3] nanocomposite
results in a remarkable increase in the photocatalytic activity of
Cr(VI).

The electron-hole division method in [Fe.sub.2][O.sub.3] was
additionally incorrigible by PL analysis. PL spectra of
[Fe.sub.2][O.sub.3] are evidently shown in Figure 11. The PL emission
peaks are observed at 416 nm and 440 nm [45, 46]. This is mostly
accredited to the effectual separation of charge carriers on the
marginal photocatalyst.

3.6. Reusability of Photocatalysts. In order to investigate the
stability and durability of the [Fe.sub.2][O.sub.3] nanomaterial
recycling experiments were conducted for the photodegradation of Cr(VI),
after the successful recycling results of X-ray fluorescence were shown
in Table 4. After the completion of each cycle, the photocatalyst was
collected using an external magnet, washed with double-distilled water,
dried overnight, and utilized. As depicted in Figures 12 and 13, the
photodegradation percentages of Cr(VI) for five successive cycles were
found to be 78.15%, 74.65%, 72.45%, 71.98%, and 70.24%, respectively; a
photocatalyst is one of the most important supplies for unbeaten
industrial applications. The [Fe.sub.2][O.sub.3] nonporous materials
exhibit 76.24% of photocatalytic activity after five successive cycles
in Figure 16 and, in Figures 14 and 15, the decrease in activity
following two cycles is credited to the loss of catalyst throughout the
easy washing process [47, 48]. In addition, there is no obvious change
observed in the XRD pattern of [Fe.sub.2][O.sub.3] in Figure 17, after
five cycles. These results indicate that the [Fe.sub.2][O.sub.3]
nanoporous materials could be used as a stable photocatalyst for the
degradation of heavy metals in industrial wastewater under sunlight as
clearly indicated in the XRF pattern, Figure 18.

3.7. Toxicity Test. The exceptional upconversion materials for
emitting visible light are the suitable candidates for both in vivo and
in vitro bioimaging as well as in photodynamic therapy. In addition to
photocatalytic activity, since the upconversion particles need to be
used in living cells, the toxicity study is also very important. The
outcome showed that the assorted sample decreases the viability of
B16-F10 cells in a dose-dependent manner as in Figure 20. As shown in 5
g/mL concentration, the lowest concentration of sample used in the assay
did not reduce the cell viability noticeably as compared to other
increased concentrations. Among [Fe.sub.2][O.sub.3] found to be low in
systematic manner, nanoporous [Fe.sub.2][O.sub.3] is the safest and the
superlative alternative in terms of toxicity.

4. Conclusion

In summary, we have demonstrated a low-cost hydrothermal method for
the preparation of a visible light responsive and magnetically discrete
[Fe.sub.2][O.sub.3]. In such an easy process, the preparation procedure
is simplified effectively, and the preparation time has been shortened
which is beneficial for industrial production. The [Fe.sub.2][O.sub.3]
nanocomposite exhibits the highest photocatalytic activity within 120
min of Cr(VI) and is degraded using 0.75 g [L.sup.-1] of
[Fe.sub.2][O.sub.3], The electron-hole separation process in the
nanocomposite has also been explained schematically. The high
recyclability of [Fe.sub.2][O.sub.3] nanoporous material is expected to
have excellent application prospects as a good photocatalyst in
wastewater treatment, which needs further investigation. Moreover, the
synthesized samples are highly stable in the photocatalytic process and
nontoxic to the living cells. Hence, the combination of high
photocatalytic stability, nontoxicity, and efficient upconversion
properties in the synthesized material paves the mode for its
application in photocatalysis, energy renovation, and the biological
area.

https://doi.org/10.1155/2018/1593947

Data Availability

The data used to support the findings of this study are available
from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest
regarding this manuscript.

Acknowledgments

The first author Abhilash M. R. (IF160104) is thankful to the
Department of Science and Technology, Govt. of India, New Delhi, for
awarding the financial assistance through DST-INSPIRE Junior Research
Fellowship to carry-out the research in the University of Mysore, Mysuru
570 006, Karnataka, India.